Heat-recovery boiler

ABSTRACT

The invention relates to a heat-recovery boiler ( 3 ) consisting of a tube bundle heat exchanger ( 7 ) which is incorporated into a pressure vessel ( 2 ) downstream of a gasification device. Displacement bodies ( 9 ) are inserted into pipes which are flowed around by hot process gas. According to said invention, in order to avoid corrosion problems like metal dusting, said displacement bodies ( 9 ) are made of graphite.

The present invention relates to a heat-recovery boiler comprising atube bundle heat exchanger permanently incorporated into a pressurevessel downstream from a gasification device, a displacement body, whichextends over at least a part of the length of the pipe, being insertedcentrally and coaxially into each of the pipes which hot process gasflows around to form an annular space with the interior of the pipe.

In the chemical industry, heat-recovery boilers are widely employed toexploit the waste heat from upstream processes for steam generation, inthat hot process gases, typically having a temperature from 800 to 1300°C., are cooled and simultaneously high-pressure steam is generated. Inthis case, deposits may form on the inner surface of the pipes, whichsignificantly impair the heat transfer between the process gas and thecoolant liquid flowing around the pipes because of their comparativelylow thermal conductivity. The formation of such deposits is to beattributed either to materials existing in the process gases or to thosematerials which first form in the pipes upon cooling of the processgases. The process of the occurrence of such disadvantageous deposits isreferred to in the professional world as “fouling”. In order to limitfouling in the pipes, providing the process gases with a sufficient flowvelocity is known. However, since the flow velocity may not be elevatedunlimitedly for reasons of a pressure loss which rises therewith,displacement bodies are inserted into the cooling pipes in a known way,which are to either elevate the turbulence of the process gas flow oreven elevate its flow velocity locally, through which the deposits ofsolids are reduced and simultaneously the heat transfer is improved. Thedisplacement bodies, which typically comprise metal and are preferablyimplemented as closed insertion tubes, are disadvantageously subject,particularly if the process gases have high concentrations of carbonmonoxide, to a significant corrosive attack, referred to as “metaldusting”, in a temperature range from 400 to 850° C., preferably 450 to750° C. Metal dusting is based on the enrichment of the matrix of ametallic material in the superficial region with carbon, carbidecompounds first arising and, upon further saturation, elementary carbonbeing precipitated. The material structure is destroyed by theprecipitation of the carbon, so that it erodes. A requirement for theerosion is the presence of a potential for carbon formation. Thispotential may be defined by the following reaction equations in thecomponents of a gas mixture obtained through gasification of carbon:CO+H₂

C+H₂O(CO reaction)2CO C+CO₂

(Boudouard reaction)

The associated equilibrium temperature may be determined for each of thetwo reactions from the composition of the gas generated by thegasification. Since both reactions run exothermically, a potential forproducing carbon exists if the gas falls below at least one of thesetemperatures as it is cooled. Whether metal dusting actually occurs isdecisively a function of the associated kinetics, whose influencingvariables are determined by the local temperature and the material. Thetemperature limit, below which metal dusting no longer occurs forreasons of kinetics, may be viewed as relatively well-established on thebasis of experience, however, it is still largely open how suitabledifferent metallic materials are for use above this temperature limit.In principle, all iron and nickel alloys are susceptible to metaldusting, however, a more or less strong occurrence of the metal dustingoccurs as a function of the further components determining themechanical-technological properties of alloys. Up to this point, thedevelopment of a material resistant to metal dusting has not beensuccessful, nor is there a sufficiently established theory about thedetailed procedures during metal dusting.

In general, metal dusting may be avoided if the displacement bodies areonly subjected to process gases whose temperatures lie above or belowthe critical temperature range of 400 to 850° C. In the heat-recoveryboiler, the pipes which the hot process gases flow through are cooled totemperatures lying significantly below 400° C. by the coolant liquidsurrounding them and the vaporization reaction occurring. Since,however, the use of displacement bodies in the critical temperaturerange of 400 to 850° C. may not be avoided under all circumstances, itis necessary to consider the risk of metal dusting when selecting thematerial for the displacement body.

It is the object of the present invention, in the initially describedheat-recovery boiler, to provide a displacement body resistant to acorrosive attack by metal dusting which is insertable into the pipes ofthe tube bundle heat exchanger.

This object is achieved according to claim 1 in that the displacementbody comprises graphite.

In order to avoid oscillation of the displacement body in the pipes,centering elements are attached to the periphery of the displacementbody, preferably materially bonded to the displacement body.

Advantageous implementations of the displacement bodies are specified inclaims 3 through 8.

According to the present invention, the displacement bodies are insertedfrom the outlet side of the process gases into the pipes and extend overat least 30% of the pipe length.

The displacement bodies are expediently made of multiple sections, whichare connected via mechanical means made of carbon, such as threaded pinsor the like.

The present invention is described in the following in greater detailand for exemplary purposes.

By reacting natural gas with steam and oxygen at a temperature of 970°C., a gas mixture is generated which is composed of 0.1 volume-percentN₂, 6.0 volume-percent CO₂, 14.5 volume-percent CO, 47.3 volume-percentH₂, 0.7 volume-percent CH₄, and 31.4 volume-percent H₂O. The gas mixturesupplied to the cracked gas boiler has a pressure of 30 bar and aneffective temperature of 9700C. An equilibrium temperature of 788° C.for the CO reduction and of 820° C. for the Boudouard reaction resultfrom the composition of the gas mixture. This means that when thetemperature falls below 820° C., a potential for metal dusting exists.In the tube bundle heat exchanger positioned in the cracked gas boiler,which is referred to as a heat-recovery boiler, the gas mixture istherefore cooled to a temperature of approximately 450° C. The gasmixture contains small quantities of components which produce solid orliquid compounds with the CO₂ upon cooling in the heat-recovery boileras a function of the temperature. Such components are typically alkalinecompounds which are introduced, for example, with the natural gas,steam, and/or oxygen or are dissolved from ceramic masses existing inthe reaction system, such as the reactor lining or catalysts. Above all,compounds containing sodium and potassium form solid carbonates uponcooling, which at least partially form deposits on the heat exchangersurfaces and therefore worsen the heat exchange, with the result thatthe process gas temperature rises to a temperature of approximately 500°C. at the outlet of the heat-recovery vessel. Since such a temperatureis harmful to the process unit downstream from the heat-recovery boiler,a closed pipe comprising a nickel-chromium alloy of the type Incone®601, which has the highest resistance to metal dusting currently known,is inserted into each of the cooling pipes as a displacement body whileforming an annular space with the interior of the cooling pipe, throughwhich the free cross-section of the cooling pipe is narrowed and theflow velocity of the process gases is elevated, so that the outlettemperature of the process gases is reduced to approximately 450° C.

However, it is a significant disadvantage that the displacement bodiespartially dissolve after a relatively short service life of only a fewweeks because of corrosion and thus significant quantities of rust-likesolid fall into the condensate of the gas generation facility. The formof the corrosion occurrence and the formation of carbon decisivelyindicate a material attack by metal dusting. In addition, the nickelreleased upon corrosion results in damage to catalysts, which maypossibly be positioned in the process unit downstream from theheat-recovery boiler.

To elevate the resistance, the displacement body comprising Inconel® 601was additionally coated with a 1.5 mm thick layer made of zirconiumsilicate as an experiment. As a result, this displacement body wassubject to a comparatively even more significant corrosive attack bymetal dusting than was the case with the uncoated displacement body.

In contrast, the displacement bodies comprising graphite used accordingto the present invention have no damage even after a relatively longoperating time of more than a year.

Displacement bodies comprising the material Inconel® 601, which werelargely destroyed by metal dusting, are illustrated in the photo shownin FIG. 1.

The diagram shown in FIG. 2 shows the course of the corrosion attack bymetal dusting along a 3000 mm long displacement body made of thematerials Inconel® 601 (dot-dash line), Inconel® 601 coated withzirconium silicate (dashed line), and graphite (solid line). It may beseen that the corrosive attack on the displacement bodies made of thematerials Inconel® 601 and Inconel® 601 coated with zirconium silicateis largest in the zones which have the highest temperatures.Accordingly, the corrosive attack falls to <0.5 mm up to the end of thedisplacement bodies. The displacement bodies comprising graphite show nochanges.

The present invention will be explained in greater detail for exemplarypurposes by FIG. 3 through FIG. 6.

FIG. 3 shows a partial longitudinal section through a cracked gas boilerin the region of the heat-recovery boiler having displacement bodiesmade of graphite inserted into its cooling pipes

FIG. 4 shows an enlarged illustration of the detail X of FIG. 1

FIG. 5 shows a cross-section along the section line D-D of FIG. 4

FIG. 6 shows a cross-section along the section line E-E of FIG. 4

By reacting natural gas with steam and oxygen, a gas mixture having atemperature of 970° C. and essentially comprising H₂, CO, CO₂, H₂O, andCH₄ is generated, which is released via the intake chamber (1) end ofthe cracked gas boiler (2) to the heat-recovery boiler (3). Theheat-recovery boiler (3) contains fixed floor pipes (4, 5) at the gasentry and gas outlet sides, into whose holes (6) the ends of the 3000 mmlong cooling pipes (7) are welded. The gas mixture exiting out of theheat-recovery boiler (3) leaves the cracked gas boiler (2) via theoutlet chamber (8). A displacement body (9) made of graphite, which ishexagonal in cross-section, is inserted concentrically into each of thecooling pipes (7) to form an annular space (10) with the interior of thecooling pipe (7). Centering elements (11), which are triangular incross-section, are attached to the lateral surface of the displacementbody (9) along a spiral line with the tip resting on the interior of thecooling pipe (7). The displacement bodies (9) have a length of 3000 mmand are inserted into the cooling pipes (7) from the side of the gasoutlet, a tubular holder (12) being screwed onto each of the end piecesof the displacement bodies (9) projecting past the floor pipes (5) onthe gas outlet side. The outlet chamber (8) of the cracked gas boiler(2) is lined with a ceramic layer (13). The displacement bodies (9) eachcomprise individual sections (14, 15), which are connected to oneanother via carbon threaded pins (16).

1. A heat-recovery boiler (3), which comprises a tube bundle heatexchanger permanently installed in a pressurized container (2) and isdownstream from a gasification device, a displacement body (9) extendingover at least a part of the length of the pipe being inserted centrallyand coaxially into each of the pipes (7), which the hot process gasesflow through, to form an annular space (10) with the interior of thepipe, characterized in that the displacement body (9) comprisesgraphite.
 2. A heat-recover boiler according to claim 1, characterizedby centering elements (11) which are located around the periphery of thedisplacement body (9) and are preferably materially bonded to thedisplacement body.
 3. The heat-recovery boiler according to claim 1,characterized by centering elements (11) attached running radially inmultiple cross-sectional planes of the displacement body (9), a passagewhich is a circular section in cross-section existing between each twoneighboring centering elements.
 4. The heat-recovery boiler according toclaim 1, characterized in that the centering elements (11) arepositioned on a line running diagonally or in a spiral to the axis ofthe cooling pipe (7).
 5. The heat-recovery boiler according to claim 1,characterized in that the displacement body (7) has the cross-section ofa circle.
 6. The heat-recovery boiler according to claim 1,characterized in that the displacement body (7) has the cross-section ofa regular polygon, preferably a hexagon.
 7. The heat-recovery boileraccording to claim 6, characterized by centering elements (11) which areequilateral triangles in cross-section, the length of the base of thecentering elements corresponding to the length of the corresponding sideof the polygon.
 8. The heat-recovery vessel according to claim 6,characterized by centering elements (11) which are equilateraltrapezoids in cross-section, the length of the base of the centeringelements corresponding to the length of the corresponding side of thepolygon.
 9. The heat-recovery vessel according to claim 1, characterizedin that the displacement bodies (9) are inserted into the pipes (7) fromthe outlet side of the process gases and extend over at least 30% of thepipe length.
 10. The heat-recovery vessel according to claim 1,characterized in that the displacement bodies (9) are each assembledfrom multiple sections connected via mechanical means comprising carbon,such as threaded pins or the like.